1. Field of the Invention
The present invention relates to a smoothing circuit used in an AGC (automatic gain control) circuit or the like for a receiver.
2. Description of the Related Art
An AGC circuit is used for control of the input signal level in AM receivers, FM receivers, and the like. In this AGC circuit, a signal is formed which changes gradually so as to follow a change in signal level when the signal level changes, and a smoothing circuit is used to form such a signal.
FIG. 8 is a circuit diagram showing the configuration of a conventional smoothing circuit. The smoothing circuit shown in FIG. 8 is formed by combining two resistors 100 and 102 and a capacitor 104. When an input voltage is applied to one end of the resistor 100, a charging current flows into the capacitor 104 through the resistor 100 and the terminal voltage of the capacitor 104 is thereby increased. If the resistance value of the resistor 100 is R1 and the electrostatic capacity of the capacitor 104 is C, time t1 during which the terminal voltage of the capacitor 104 increases to a predetermined value can be expressed by R1×C. This time t1 is an attack time. Time t1 is set to about 10 to 50 msec in a smoothing circuit used in an AGC circuit for a receiver or the like.
When application of the input voltage to the one end of the resistor 100 is stopped, the capacitor 104 is discharged through the resistor 102 and the terminal voltage of the capacitor 104 is thereby reduced. If the resistance value of the resistor 102 is R2, time t2 during which the terminal voltage of the capacitor 104 decreases to a predetermined value can be expressed by R2×C. This time t2 is a release time. Time t2 is set to about 200 to 500 msec in a smoothing circuit used in an AGC circuit for a receiver or the like.
The above-described conventional smoothing circuit has a problem that in the case of realizing an attack time of about 10 to 50 msec and a release time of about 200 to 500 msec, the device constant of each of the resistors 100 and 102 and the capacitor 104 is so increased that it is difficult to limit the size of the circuit or to form the circuit in an IC, because it is necessary to set a large time constant in each of the combination of the resistor 100 and the capacitor 104 and the combination of the resistor 102 and the capacitor 104. For example, in the case of formation in an IC, the resistance value of the resistor actually formable is at most about 500 kΩ. If such a resistor is used and if the release time t2 is set to 100 msec, C=t2/R2=0.2 μF. However, the electrostatic capacity of a capacitor in an IC according to a design considering the manufacturing cost, etc., is 20 pF or less. After all, it is difficult to form the entire smoothing circuit in an IC, and the capacitor heretofore used is an externally-mounted large capacitor.